The present invention relates to optical communication systems and, more particularly, to an optical signal transmission method in which a compensation of the chromatic dispersion of the line is effected by means of compensation modules with a nominally linear characteristic (specifically, chirped optical fiber gratings). The invention also relates to a communication system using such a method.
Silica fibers of different characteristics are the most commonly used transmission lines in optical fiber communication systems. Among such fibers, the so-called conventional single mode fibers, which are widespread all over the world and make up more than half of all installed lines, substantially show no chromatic dispersion at wavelengths within the so-called xe2x80x9csecond transmission windowxe2x80x9d (namely wavelengths around 1.3 xcexcm).
In order to compensate optical signal attenuation due to propagation along the fiber, use is increasingly being made of optical amplifiers, which obviate the need for optical-to-to=electrical conversion and vice versa at the repeaters. The most commonly used optical amplifiers show, however, their highest operation efficiency at wavelengths within the so-called third transmission window (i.e. wavelengths around 1.5 xcexcm). At such wavelengths the conventional fibers have their minimum attenuation, but exhibit a considerable chromatic dispersion, in the order of 15-20 ps/nm/km. Consequently, a strong pulse broadening originates and this is a hindrance to long distance high bit rate transmissions.
In particular, chromatic dispersion effects increase as signal bit rate increases and have a great influence on NRZ signals already at bit rates of about 10 Gbit/s. As a matter of fact, under these conditions, which are of immediate interest to operators, chromatic dispersion limits the maximum length of a link to about 70 km on conventional fibers. For longer links it is therefore necessary to make use of chromatic dispersion compensation techniques.
Among such techniques, use of chirped optical fiber gratings (i.e. gratings in which the pitch is variable) appears to be greatly attractive, since such gratings allow use of compensation modules which are compact and exhibit low insertion loss.
A chirped optical fiber grating may be compared to a sequence of constant pitch gratings, each reflecting a given wavelength and transmitting all other wavelengths. In general, the pitch linearly varies along the grating. A compensation module using such a grating can be connected to the optical fiber transmission line so that the different wavelengths within the signal to be compensated travel along the grating, are reflected and are reinserted into the line with different transmission delays. This can be exploited to compensate chromatic dispersion. To achieve this compensation it is necessary to choose such a pitch behavior that the grating has a group delay t which is linearly dependent on optical frequency xcfx89 in complementary manner to group delay in the fiber.
However, chirped gratings usually show a group delay vs. optical frequency characteristic that is not perfectly linear due to technological reasons (in particular, owing to the impossibility of having gratings of infinite length and to discontinuities resulting from pitch variations). It has been demonstrated that the presence of ripples in the characteristic xcfx84(xcfx89) (i.e. of fluctuations around the linear behavior) affects system performance. In particular, these effects depend on the ripple period and are higher when the ripple period (in terms of optical frequency) is close to the bit rate of the transmitted signal. These ripples, even if constant in time, cannot practically be foreseen and their impact on transmission system is strictly dependent on the optical carrier frequency which, as known, may in turn fluctuate around its nominal value. Thus their compensation is practically impossible.
The only way to reduce this effect is therefore to try to manufacture gratings in which said ripples are kept as limited as possible. Nevertheless, it must be considered that, in order to keep manufacturing costs limited, there is a trend to manufacture gratings which compensate chromatic dispersion in line spans of a predetermined length, usually less than 100 km. Thus, in order to compensate the chromatic dispersion over a rather long line, it may be necessary to resort to a number of gratings and their individual effects are likely to cumulate.
The interest in a method of reducing the influence of those ripples is therefore obvious. No solutions to this problem are available from literature so far.
The invention is based on an accurate analysis, carried out by the inventor, concerning the behavior of an optical communication system where chromatic dispersion compensation is carried out by means of gratings. This analysis has shown that the effect of ripples in the characteristic xcfx84(xcfx89) of the grating(s) on the system not only depends on the global characteristics of the grating(s) (i.e. amplitude and frequency of said ripples), but also on parameters relating to the transmitter, to the line amplifiers and to the fiber. In particular there are configurations showing a much higher tolerance to the grating defects.
According to the invention, a method for optical signal transmission is provided wherein, in order to minimize the influence of the fluctuations around a nominally linear behavior of the delay group vs. optical frequency characteristic xcfx84(xcfx89) of a compensation module, a curve representative of the system performance degradation due to chromatic dispersion is determined and, for a given line span length, the compensation is effected so that the operating point along the degradation curve is kept within a region of limited slope of said curve
As will be shown, this solution is based on the fact that, according to what has been highlighted by the inventor, the impact of grating defects can be seen as a compensation fluctuating around an ideal value.
Advantageously, the compensation module is based on the use of chirped optical fiber gratings.
The degradation curve may be, for instance, a curve representing the power penalty with respect to a reference condition (that is, a curve representing the eye diagram closure) of the transmission system when varying the fiber length on which dispersion compensation is carried out.
The present invention also provides an optical fiber transmission system using the method, wherein, for chromatic dispersion compensation, use is made of at least one compensation module with a delay group vs. optical frequency characteristic which is nominally linear, with such a slope that the compensated line length brings the system operating point in correspondence with a minimum (or at least in a region with limited slope) of a curve representing the system performance degradation due to chromatic dispersion.